I didn't even know that Gastrotheca sp. were still in the hobby! Those are some amazing frogs! How old are they? Also, if you post some pictures from the back of the frog, there might be some evidence of maternal pouches developing. Just my opinion, but I think the most likely of these frogs to be female is the middle and/or the first one.
Are there any other species that you know of that are still in the hobby?
You are so lucky to have these frogs!! This may be my favorite genus of amphibians! (Hence my username)
By the way, great-looking frogs! Have you had any breeding activity from them? I do not know much about sexual dimorphism with these frogs, however, I can tell that the smallest frog is darker and likely a male. This corresponds with the sexual dimorphism of other frog species too. For instance, most Ranids exhibit severe variation of size within males and females. So do most bufo-toads. It is pretty easy to see which gender a ranid or bufo-toad is by the age of about 2 (sometimes earlier).
Do you know of any other species that are kept or have been kept in the hobby?